1. Field of the Invention
The present invention relates to method and apparatus for effecting at least one of the recording and reproduction of information, by irradiating an optical recording medium with a laser beam.
2. Related Background Art
In a conventional recording and reproducing apparatus, there has been generally used such a method that the power of a laser beam to be radiated onto an optical recording medium would be changed between upon recording and reproducing. That is, recording of information is effected by such a manner that light from a semiconductor laser is irradiated with an energy level sufficient to change the reflectivity at a portion where a light spot is irradiated, e.g., necessary for causing a dissolution. For reproducing information on the above recording medium, the light is irradiated from the semiconductor laser at a sufficiently low energy level not to change the reflectivity, in which the information was read out from changes of the reflected or transmitted light with the light spot tracing an information recorded portion. Therefore, an optical information recording medium is required to have a development characteristic with a distinct threshold as indicated by a dot-and-dash line a in FIG. 1. However, in practice, a recording medium with a dye system having significant durability in temperature and humidity has a characteristic as shown by a solid line b in FIG. 1, and it cannot satisfy the above-mentioned requirement. Note that FIG. 1 shows the applied energy density in the axis of the abscissa, and the change of reflectance in the axis of the ordinate, i.e., the formation of pits. As clearly seen from the figure, an actual characteristic curve indicates that the applied energy density during reproducing of information on a recording medium must be significantly small. If the reproduction of information is effected at an energy level with which any reflectance may be changed, as a result of accumulation of the changes, the reliability on the reproduction of information may be decreased, due to occurrence of reading errors.
Under such a limitation, the quantity of light incident upon a photodetector for detecting the change of light quantity will be naturally reduced. Therefore, it becomes difficult to detect an automatic focusing error signal (hereafter called an AF signal), an automatic tracking error signal (hereafter called an AT signal), and a reproduction signal.
Generally, when an optical disk is used as a recording medium, the pulling-in loop for the automatic focusing (AF) and the automatic tracking (AT), and the selection (access) of an information track are performed in a rotating state of the optical disk. Thereby, with an increased rotation speed, a portion of the light energy irradiated onto the recording medium can be effectively reduced. This is why if the light quantity of radiation is the same, the light energy density on a radiated portion of a moving medium is less than that of a medium in the stationary state. However, the setting of a rotation speed is largely restricted from the viewpoint of higher-density recording.
In particular, when a card-like optical recording medium (optical card) is used as a recording medium, the AT and AF pull-in operation, and the access operation are more preferably performed in the stationary state of the optical card, in order to maintain the reliability of attachment. However, it is difficult to prevent the radiation time of an optical spot from being longer. That is, the recording and reproducing are effected by reciprocating the optical card relative to a laser beam, in which the moving speed is naturally limited because of reducing the speed up to the stationary state at the end of the reciprocatory motion. In addition, when nothing is done, the optical card is placed in a stationary waiting state, in which AF and AT must remain operated. This is because, once AF and AT are made inoperable, it takes some time to loop in a servo mechanism again. And in this case, a light spot is applied to the same position for a long time, so that the applied energy density will be increased with the passage of time, thereby causing the change of reflectance. This will be described with reference to FIG. 1 in the following. In FIG. 1, a symbol E.sub.w indicates the applied energy density during recording for formation of pits, and E.sub.r the applied energy density at other states, i.e., the reproduction, the access, and the waiting states. The recording of a pit is made onto one position of an optical card in a short time. The variation in the medium with E.sub.r is very small. But as the reproduction of information is often repeated, thereby causing the variation in the medium to be accumulated, there is a fear that the information may be diminished during the repeated reproduction of information. Also, in the waiting state, as the light radiation is made on the same position for a long time, and so the variation in the recording medium is accumulated, pseudo pits may be formed. Accordingly, E.sub.r is desirably as small as possible.
By the way, the applied energy density is proportional to the emission intensity of a laser light source, for example, a laser diode, and the applied time on the same position of an optical card, and is inversely proportional to the area of a light spot on the optical card. Thus, it is conceived that the applied energy density is substantially reduced by changing the area of the light spot depending on the operation state. However, this method is difficult to use in practice, because an optical system must be activated in every state. Another method can be conceived in which the applied energy density is substantially reduced by increasing the recording speed. With this method, in recording the information, a medium must be given a sufficient applied energy density to form stable pits by increasing the emission intensity of the laser diode. In other words, a difference between the emission intensity during recording of a pit and that during other cases, e.g., the reproduction or the reading of AT and AF signals, must be increased. However, as is well known, the ratio of the emission intensity of the laser diode cannot be increased beyond a certain value. FIG. 2 shows a general characteristic of the light power to the input current of a laser diode. As clearly seen, the output of the laser diode is not the laser emission up to a threshold current I.sub. TH, but is the LED emission. Only when the input current is above I.sub.TH, does laser emission occur with the maximum rated light output P.sub.MAX obtained at I.sub.MAX. That is, a range where the laser diode can be used as a laser light source is from P.sub.MAX to the light output P.sub.TH at I.sub.TH. In a normal laser diode, the ratio between P.sub.MAX and P.sub.TH is at most about 20:1, and accordingly, in order to prevent the degradation due to the reproducing light, i.e., the variation in a recorded pit, it is necessary to suppress the light intensity on an optical card at P.sub.TH to a small value, by attenuating the intensity with the optical system. Correspondingly, as the light intensity on the optical card at P.sub.MAX is also attenuated, the recording speed must be reduced.
FIG. 3 shows that from a characteristic of variation in an optical recording medium as shown in FIG. 1, a light output characteristic of a laser diode as shown in FIG. 2, and a laser diode control signal, the variation in the recording medium for the above control signal can be derived. FIG. 3 shows the laser diode control signal in the fourth quadrant, with the time t being in the axis of the abscissa and the laser diode driving current I being in the axis of the ordinate. And it shows that the current I is transferred from a reproduction level I.sub.r to a recording level I.sub.w, and again to I.sub.r. When the light spot is scanning an optical card at an equal speed, the axis of abscissa t is equivalent to the distance x on the optical card. The third quadrant is a characteristic curve of a laser diode, where I.sub.r, I.sub.w are equivalent to light intensities P.sub.r, P.sub.w, respectively. The second quadrant shows a recording medium characteristic, like in FIG. 1. Here, as the light intensity is proportional to the applied energy density with the same scanning speed and the same light spot area, P.sub.r, P.sub.w become E.sub.r, E.sub.w, respectively, and E.sub.r, E.sub.w become D.sub.r, D.sub.w in the second quadrant, respectively. Accordingly, in the first quadrant, a graph can be obtained in which the distance x is indicated in the axis of abscissa, and the degree of variation in the recording medium in the axis of ordinate. As clearly seen from the figure, the variation corresponding to D.sub.r other than recording pits on the recording medium will be generated due to a small ratio of P.sub.w to P.sub.r.